Pilot plant study to assess the fate of two volatile methyl siloxane compounds during municipal wastewater treatment

The environmental fate of octamethylcyclotetrasiloxane (D-4) and decamethyl cyclopentasiloxane (D-5) during municipal wastewater treatment has been inv estigated in an activated sludge pilot plant by continuously dosing the inf luent with exagerated amounts of the compounds and measuring their emission s in the off-gas, effluent, and sludge streams. The total removal was found to be 86.4 +/- 3.9% for D-4 and 95.8 +/- 1.5% for D-5 (+/-1 SD statistical uncertainty). However, low overall mass balances were found for both compo unds in the pilot system. In order to elucidate the detailed removal mechan isms for these two compounds, the model TOXCHEM was used to simulate their removal and analyze their mass balances in the pilot system. Experimental a nd model results indicated that the low mass balances were caused by the si gnificant underestimation of the primary sludge removal by the grab samplin g method used for sludge analyses. This underestimation maybe due to the hi gh variability of the primary influent suspended solids concentration and h ence inhomogeneity in the primary sludge underflow. When this underestimati on was taken into account, primary sludge removal and volatilization in the aeration basin were found to contribute approximately equally to the remov al of D-4 and D-5 in the pilot plant system. The present results are useful for the environmental risk assessment of these two siloxane compounds in t he unlikely event of their point source discharge into municipal wastewater systems.